you can report bugs for Gnash in this link gnu.org/software/gnash/bugs.html and about your problem I don't understand are you bugs did from use Gnash only or not you try to use flash player from adobe it's working fine
–
Mahmoud20070Feb 18 '12 at 12:30

English translation (with the help of Google Translator, plus formatting and clarity changes):

Here's the solution, when the problem is that the videos worked before the upgrade, but not after.

Run in terminal:

gksu gedit /usr/bin/firefox

That opens the Firefox executable script with a word processor, with permission to modify it. Below the first line of the file where it says #!/bin/sh, you must add the line:

export XLIB_SKIP_ARGB_VISUALS=1

That should be the second line of the file. Save the changes to the file, close it and restart Firefox, and you should be able to re-watch the videos.

Firefox was rewritten to update the browser settings but no video options such as transparency, which makes the plugin fails to render the image. That line, once added, accounts for those transparencies.